The FT4 Protocol for Digital Contesting by K1JT

Introduction: FT4 is an experimental digital mode designed specifically for radiocontesting. Like FT8, it uses fixed-length transmissions, structured messages withformats optimized for minimal QSOs, and strong forward error correction. T/Rsequences are 6 seconds long, so FT4 is 2.5 × faster than FT8 and about the samespeed as RTTY for radio contesting. FT4 can work with signals 10 dB weaker thanneeded for RTTY, while using much less bandwidth.Basic parameters: FT4 message formats are the same as those in FT8 and encodedwith the same (174,91) low-density parity check code. Transmissions last for 4.48 s,compared to 12.64 s for FT8. Modulation uses 4-tone frequency-shift keying atapproximately 23.4 baud, with tones separated by the baud rate. The occupiedbandwidth (that containing 99% of transmitted power) is 90 Hz. Threshold sensitivity for50% decoding probability is S/N = –16.4 dB, measured in the standard 2500 Hzreference noise bandwidth. A priori (AP) decoding can push threshold sensitivity downto –18 dB or better.

Installation and Initial SetupTo join the FT4 test group and participate in one or more upcoming “mock contest”practice sessions, follow these steps to install and configure a release candidate forWSJT-X 2.1.0:1. Download the installation package from a link to be provided.2. Install the program in the usual way for your operating system. To avoid conflictwith your normal operation, you may wish to use a different installation directoryfrom that used for WSJT-X 2.0.3. Start the program as you normally would for FT8.4. From the Configurations menu select FT8 | Clone (or Default | Clone).5. Rename the resulting new configuration to FT4, and select the new configuration.6. Select FT4 from the Mode menu.7. On the File | Settings | Frequencies tab, right-click any line in the WorkingFrequencies table and press the Reset button. This action will pick up therecommended working frequencies for FT4.8. Check the box Special operating activity on the Settings | Advanced tab.Then select RTTY Roundup messages and enter your proper contest exchangein the field RTTY RU Exch.9. Instructions for connecting WSJT-X to N1MM Logger+ can be found in theWSJT-X 2.1 User Guide, Section 4.5.2You now have a release candidate for WSJT-X 2.1.0 configured for FT4 operation. Youmight find it useful to download and examine an FT4 sample file. Select DownloadSamples from the Help menu and tick the FT4 box. Configure the Wide Graphcontrols as shown in Figure 1 and select Deep on the Decode menu, then open thesample file from the File menu. The recorded interval should produce 19 decodes, asseen in Figure 1. Most of the decoded signals are inaudible or barely audible in theaudio file. Fewer than one-third are strong enough to be reliably decoded if they hadbeen RTTY signals — and only then if they were spread out over a much widerfrequency range.

Operation: Aside from its much shorter T/R sequences, FT4 behaves essentially thesame way as FT8. One new on-screen control is available for FT4 operation, a buttonlabeled Best S+P, just below the Call 1st checkbox. Clicking this button during an Rxcycle arms the program to examine all CQ messages decoded at the end of the 6-second Rx sequence. The program will select the best potential QSO partner (from acontesting perspective), and treat it as if you had double-clicked on that line of decodedtext. Here “best potential QSO partner” means “New Multiplier” (1st priority) or “NewCall on Band” (2nd priority). “New Multiplier” is currently interpreted to mean “NewDXCC”; a more broadly defined multiplier category (for the ARRL RTTY Roundup rules)will be implemented soon. We may also provide additional priority rankings, forexample “New Grid on Band” (useful for North American VHF contests), sorting bysignal strength, etc.For keyboard control of transmitted messages, check the box Alternate F1–F6bindings on the Settings | General tab. In typical contest-style operation you can thenhit function key F1 to solicit a QSO by sending CQ. To respond to a CQ and send yourcontest exchange, double-click on the decoded message. Alternatively, you can click onBest S+P and let the selection algorithm choose a station to call. Auto Seq and Call1st checkboxes behave as in FT8, and thus the remainder of a minimal QSO cancontinue without further operator action. Function keys F2 – F5 may be used to sendmessages displayed in entry fields for Tx2 – Tx5 on tab 1, at bottom right of the mainwindow. Function key F6 toggles the checked status of Call 1st, and key combinationAlt+B can be used to toggle the armed status of Best S+P.FT4 is presently configured so that a station operating in Search-and-Pounce (“S+P”)mode logs a QSO when RR73 is transmitted, and the CQing (“Run”) station logs a QSOwhen RR73 is received. Like FT8, FT4 makes little distinction between an S+P stationand a Run station. An operator can switch easily and frequently between these twoways of initiating QSOs, and contesting skill will depend on optimizing these and manyother operating decisions. With a steady stream of available stations to work, singleradio QSO rates well above 100/hour are possible using FT4.

Transmitted signal: FT4 uses a modulation technique known as Gaussian frequencyshift keying, or GFSK. The generated audio waveform consists of 105 symbols (tones)sent in sequence at one of four frequencies. The encoded series of distinct tones forpart of a transmission might originally look like the upper (red) curve in Figure 2.However, in FT4 the sequence of frequencies is smoothed by convolution with aGaussian function before being sent to the software modulator. The blue curve showsthe corresponding smoothed sequence of frequencies actually sent to the modulator.The transmitted sequence no longer has any stepwise discontinuities.Differences between the red and blue curves seem rather small, but spectra of theresulting audio waveforms are remarkably different. Figure 3 shows spectra for an FT4signal (blue) and a standard continuous-phase FSK sigal (red) for the same encoded bitsequence. The GFSK spectrum has steep skirts, occupying a bandwidth of only 75 Hzat –6 dB, 200 Hz at –60 dB, and 260 Hz at –80 dB. No additional filtering is applied tothe audio waveforms.

Figure 3 also shows the spectrum of a standard RTTY signal (purple), which is morethan 2000 Hz wide at –60 dB. Some ham software generating RTTY signals by audiofrequency shift keying (AFSK) offers optional filtering of the waveform to reduce theunwanted keying sidebands. However, such post-modulation filtering necessarilydestroys the constant-envelope nature of the continuous-phase FSK signal and leavesbehind a waveform that must be amplified with good linearity to avoid generating newunwanted sidebands. In contrast, the GFSK signal of FT4 has a constant envelope andis immune to inter-modulation distortion. 5Decoding and Frequency Usage: The FT4 decoder in WSJT-X 2.1 identifies anddecodes signals anywhere in a passband up to 5 kHz wide. As in other WSJT-Xmodes, received messages are “all or nothing” — there are no partial decodes, andfalse decodes are rare. Subtraction of decoded signals from the received data streamenables decoding of transmissions that overlap in frequency with other, possibly muchstronger signals. The last three decodes shown in the Band Activity window in Figure 1are examples of such second-pass decodes. With the normal odd/even sequencing oftransmissions and signals spaced at intervals 120 – 150 Hz, as many as 50 stations canoperate in a 3 or 4 kHz passband with little interference, even when signal strengthsdiffer by as much as 60 dB.Experience will tell what may be the best strategy for selecting dial frequencies duringcontests, and for how many ~3 kHz segments should be used for FT4 on each band.As initial guidelines we suggest the following default dial frequencies for FT4: 3.595,7.090, 10.140, 14.140, 18.104, 21.140, 24.919, 28.180, 50.318, 144.170 MHz. We willwelcome any feedback that could lead to better frequency choices.Sensitivity: Figure 4 presents measurements of FT4 decoder sensitivity over a widerange of simulated propagation conditions. Paired numbers at the right give theDoppler spread (Hz) and two-path differential delay (ms) for various sets of ITUstandard conditions, using the Watterson model for ionospheric propagation. The leftmost solid curves correspond to common mid-latitude propagation paths in undisturbedconditions. Dashed curves show the measured sensitivities for decoding with maximuma priori information, in two extremes of propagation conditions. For a rough comparisonwith RTTY, we note that in simulated mid-latitude moderate conditions RTTY has beenshown to yield character error rates less than 10% only when SNR exceeds –1 to +4dB, depending on the software modem in use. FT4 offers a sensitivity advantage ofabout 10 dB over RTTY

Previous testing: Early release candidates of WSJT-X 2.1.0 enabled us to explore theadvisability of making FT4 an asynchronous mode with no fixed start times for T/Rintervals. These tests served to highlight the significant advantages of using timesynchronized sequences. Fixed sequence lengths with prescribed start times ensurethat a much larger fraction of received signals are decodable, especially in contest-likeoperating conditions where a given station may transmit roughly half the time. In termsnormally used for communications engineering, synchronization increases channelcapacity.Schedule: A few parameters and operating behaviors of FT4 are still being tested andoptimized. It will be very useful to hold several more mock contest practice sessions,with a larger group of active participants. Even if these reveal no serious bugs orinadequacies, we think FT4 is too new to be used in two upcoming events: the ARRLVHF Contest (June 8-10) and ARRL Field Day (June 22-23). Consequently we plan tobuild a release candidate WSJT-X 2.1.0-rc5 that will “time out” on June 7, 2019. As faras possible, we plan to adhere to the following schedule:• April 22: Public announcement about FT4, with a link to this document• April 29: Second announcement, with links to downloadable installation packagesfor WSJT-X 2.1.0-rc5• May 9, 0000 – 0100 UTC: FT4 practice session, 7.090 MHz• May 14, 0000 – 0100 UTC: FT4 practice session, 7.090 MHz• June 5, 0000 – 0100 UTC: FT4 practice session, 7.090 MHz (if needed)• July 15: General Availability (GA) release of WSJT-X 2.1.0Random concluding thoughts: FT4 is a special-purpose mode designed for rapid-firecontest QSOs. It serves this purpose very effectively, but like FT8 the mode is notuseful for more extensive conversations. FT4 uses much less bandwidth than RTTYand provides reliable decoding at much lower signal levels. It has no need for “SuperCheck Partial” or similar contesting aids, and skilled operators using FT4 will find lessmotivation to use a DX Cluster or other non-radio aids. All information necessary toscore well in a contest can be obtained over the air, during the contest, through one’sown antennas and radios. With FT4 there is little distinction between CQ and S+Poperation, so it’s easy to switch frequently between the two ways of finding QSOpartners. Stations using low power and compromise antennas can participateeffectively in a contest using FT4.